1. Field of the Invention
The present invention relates to a method of producing a thin film photoconductor and particularly, to a method of producing a thin film photoconductor which is characterized by the use of amorphous silicon as a photoconductive material under such conditions that the thin film is suitable for use as a photoconductive element.
2. Description of the Prior Art
Photoconductive elements are generally employed as the light sensitive plate in electrophotographic elements and photoconductive cells. Those photoconductors which are prepared using crystalline or amorphous silicon as a photoconductive material are of particular interest because the silicon film itself is not poisonous, as opposed to, for example, selenium, CdS and the like.
Typical processes for producing photoconductors using elemental silicon include the step of drawing a single crystal ingot from a silicon melt according to Czochralski's process or a ribbon crystal growth process, and subsequently slicing or cutting the resulting single crystal in thin leaves. However, since these processes require a good deal of thermal energy in the course of the above-described proceedings, the production of a silicon film is expensive and consequently, an elements having large areas cannot be produced.
On the other hand, some processes for producing silicon photoconductors having a large area have been known. For instance, in Japanese Patent Application (OPI) No. 122471/'77 a process is described in which a thin film of amorphous silicon is produced using a silane gas by a glow discharge decomposition process. In this process, it is possible to produce thin films of silicon photoconductors having areas larger than those of conventional single crystal wafers. However, this process requires special precautions, because toxic silane gas is used in the production and further, it is necessary to maintain the temperature of the base plate at a relatively high temperature ranging from about 250.degree. C. to 350.degree. C. at the time of thin film formation. Such being the case, problems remain from the standpoint of preventing environmental pollution and reducing costs.
Another process for producing a large-area thin film of amorphous silicon is described in Solid State Communication, Vol. 20, pp. 969-972 (1976), in which the thin film of amorphous silicon is produced through high frequency sputtering of elemental silicon under an atmosphere containing argon and hydrogen gases. This process does not require the use of any poisonous materials but optimum conditions for producing the thin film of silicon are still not established. Notwithstanding the admonition that careful determination of conditions as optimum for producing excellent photoconductors will be required in the high frequency sputtering process, the aforementioned reference lacks description regarding the influence of changes in the temperature of the base plate and the partial pressure of hydrogen gas on the characteristics of the photoconductor. Usually, a semiconductor must not only have excellent photoconductivity but also a large difference between photoconductivity and dark conductivity to be suitable as a photoconductor element. Accordingly, when a thin film of amorphous silicon is produced for use as a thin film photoconductor, it is most important to establish such conditions as endow the film with the above-described characteristics.